It is essential for a good efficiency of a gas turbine or an aeroplane engine that sealing gaps between the turbine blades and the housing are made very small. Under adverse operating conditions a scraping between the blade tips and the inner surface of the housing can result. For this reason "abradables" have been developed as a coating of the housing surface. In the event of a scraping, the blade tips release fine particles from the abradable layers without their being damaged thereby. The abradable layers are manufactured in particular by thermal spraying on of ceramic powders (e.g. "HVFS" or "APS", i.e. high velocity flame spraying or atmospheric plasma spraying). It is known that the thermal expansion of a ceramic coating is substantially less than that of a coated metallic basic body. Therefore, due to the high operating temperatures, suitable intermediate layers ("bond coats") must be provided which counteract a breaking off of the coating.
Similar problems are present in combustion chambers of gas turbines, in inlet segments or in guide blades. Heat insulating coatings of a ceramic material must be suitably anchored on a surface to be screened off.
Thick ceramic coatings adhere to a basic body as a rule only when a bond coat has been previously applied to it. Such bond coats often contain oxidisable components. A ceramic coating of zirconium oxide is permeable to oxygen ions at high temperatures. Therefore an oxidation of the bond coat can result which--if it is combined with a volume increase--can lead to a breaking off of the coating.